1. Field of the Invention
The present invention relates to a diffractive optical device, especially a diffractive optical device preventing reduction in the diffraction efficiency even in an area having a small grating period.
2. Description of the Related Art
A diffractive optical device, which utilizes diffraction of light, has a grating pattern for diffraction. The grating pattern is formed by arranging a plurality of grating elements on a substrate. The diffraction efficiency, which is the ratio of light which can be diffracted with respect to the light incident on the diffractive optical device, is determined by the grating pattern. Generally, how high the diffraction efficiency can be is a matter of a prime importance in determining the quality of the diffractive optical device.
One of conventional diffractive optical devices is a diffractive microlens used for diffracting light which is incident thereon vertically. Briefly referring to FIGS. 1 and 2, such a conventional diffractive microlens 100 will be described. FIG. 1 is a plan view of the microlens 100 illustrating a grating pattern thereof, and FIG. 2 is a cross sectional view of the microlens 100. The microlens 100 includes a substrate 11. Light which is incident vertically on a bottom surface of the substrate 11 is collected or collimated above the substrate 11. As is shown in FIG. 1, a plurality of grating elements 18 are arranged concentrically on a top surface of the substrate 11 to form a grating pattern. A period at which the grating elements 18 are arranged (hereinafter, referred to as a "grating period") becomes progressively smaller toward the outer periphery of the substrate 11. As is shown in FIG. 2, each grating element 18 has a rectangular cross section.
FIG. 3 is a cross sectional view of another diffractive microlens 200 proposed by J. Jahns and S. J. Walker in "Two-dimensional array of diffractive microlenses fabricated by thin film deposition", Applied Optics Vol. 29, No. 7, pp. 931-936 (1990). The microlens 200 includes a substrate 11 and a plurality of grating elements 28 arranged on a top surface of the substrate 11. Each grating element 28 has multiple discrete phase levels. In the example shown in FIG. 3, each grating element 28 has four phase levels including the top surface of the substrate 11. Adopting such a way of counting, each grating element 18 in the microlens 100 in FIG. 2 has two phase levels. This way of counting the phase levels of the grating elements will be used throughout this specification.
While the diffractive microlens 100 has a diffraction efficiency of 41%, the microlens 200 has a diffraction efficiency of as high as 81%. It has been found that the larger the number of phase levels of the grating element is, the higher the diffraction efficiency is. For example, the diffraction efficiency is 95% where each grating element has eight phase levels, and the diffraction efficiency is 99% where each grating element has 16 phase levels.
Considering the above-described relationship between the diffraction efficiency and the number of phase levels, it is easily assumed that any type of diffractive optical devices show such relationship.
In the case that light is incident at an angle which is offset with respect to the vertical direction to the substrate, it is true that the larger the number of phase levels is, the higher the diffraction efficiency is in an area where the grating period is relatively large. The researchers including the inventors of the present invention have found that the diffraction efficiency is significantly reduced as the number of the phase levels is increased in an area where the grating period is relatively small, namely, proximate to the wavelength of the incident light.
Further, in an area where the grating period is small, precision processing is difficult to perform. Accordingly, it is substantially impossible to process the grating elements into a desirable shape, which reduces the optical characteristics of the diffractive optical device.